I'm in preliminary stages of planning a coaster trip (Cedar Point, SFGA, Mt. Olympus, and Valley Fair).

I'm planning it for early June 2008.

How crowded will SFGA be on a Saturday in early June?

A friend who is going on the trip with me said that Mt. Olympus is very crowded on weekends with fairly poor line management and wanted to put Mt. Olympus on a weekday. To do that, SFGA will have to be on Saturday.

My two choices are: SFGA on Fri and Mt. Olympus on Sun or SFGA on Sat and Mt. Olympus on Monday.

Any information from frequent visitors to these parks would be fabulous.

Why not SFGAm on Sun and Mt. Olympus on Monday? If you have something planned inbetween, I understand. But consider doing SFGAm on Sunday, drive halfway that night, which is around Lake Mills or Johnson Creek WI, and drive the rest on Monday morning. Which is similar to what I did 2 years ago on vacation.

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My top 3 coasters: Millenium Force, Dominator (RIP), Flight of Fear.

SFGAm shouldn't be too bad. But you never know. Obviously, a lot hinges on weather conditions. The closer it is to 75*, the more people they'll get into the park. Higher than that and the water rides tend to soak up a good chunk of the lines (no pun intended).
I can't say how crowded Olympus will be, but your friend is right about line management. You'll be shocked. My advice would be to get the credits and spend the rest of your day at Timber Falls (which you must visit, there's no excuse for not going to Timber Falls if you go to Olympus - it's literally right down the street).
It's interesting that you chose those four parks. Cedar Point makes sense, but Valleyfair! is sort of out of the way. I'm assuming your plan is to do Cedar Point first, then these two parks, then VF!? I would save Cedar Point for the trip home and start with GAm. Then go up to MN and hit VF!, MOA, and Como Town before heading to the Dells on your way to Cedar Point.

Also, this needs to be under Travel Tips.

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Well, Cedar Point is first and I'd rather not do CP on Friday. SO my Schedule is shaping up to be....

Weds: Fly into Cleveland
Thurs: Cedar Point
Friday: Travel day to Michigan
Sat: SFGAm
Sun: Drive to The Dells/Do Dell stuff.
Mon: Mt. Olympus
Tues: Valley Fair
Weds: Go home

Note: The drive halfway at night and halfway the next morning doesn't really work for me. And I'm past the point where I'm going to go to a park, drive 3 or 4 hours to the next park, and go to the next park the following day. I've done that one too many times.